Degradable taggant and method of making and using thereof

ABSTRACT

The present invention is a low-cost, easily deployed, degradable taggant that can be dispersed over a wide area to serve as a witness to activity in the area and for queuing of other sensors. The taggant enables nearly real-time change detection within the treated area using one or more simple optical sensing techniques.

BACKGROUND

Traditional warfare, with accepted rules of engagement, appears to havegiven way to unconventional, asymmetrical warfare. One asymmetric threatthat is proving difficult to counter and defeat is the improvisedexplosive devise (IED), commonly known as the “roadside bomb.” Today,IEDs are the major cause of combat casualties. They are the mosteffective way to cause the most harm at the least cost and are alsooften augmented with conventional mines on routes and soft shouldersthat are vulnerable to surface-laid and dug-in anti-vehicle andantipersonnel mines.

In the past, IEDs were typically laid at night, often in craters left bypreviously detonated IEDs, and used a myriad of triggering techniques:cell phones or pagers, garage door openers, pressure plates or strips,etc. Today, this is no longer the case. Hand triggered IEDs are laid atall hours of the day in order to provide the triggerman with ampleillumination for target acquisition and ordnance detonation. To beeffective, IED detection technologies and strategies must allow forday/night and low visibility operations.

SUMMARY

The present invention is a low-cost, easily deployed, degradable taggantthat can be dispersed over a wide area to serve as a witness to activityin the area and for queuing of other sensors. The taggant enables nearlyreal-time change detection within the treated area using one or moresimple optical sensing techniques.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 a is a graph of the absorption and fluorescence spectra oftetraphenylporphyrin (TPP);

FIG. 1 b is a graph of the absorption and fluorescence spectra ofindodicarbocyanine (C5);

FIG. 1 c is a graph of the absorption and fluorescence spectra ofindotricarbocyanine (C7);

FIG. 2 is a photograph of SiO₂—C₁₈ with Rhodamine 6G in the presence ofwater droplets where the powder forms a coating on the droplets, butdoes not disperse in the water;

FIG. 3 shows a schematic of fluorophores assembled with non covalentlinkage between molecules;

FIG. 4 is a photograph of a modified cuvette shown where the sample iscontained in an inner tube within a conventional cuvette;

FIG. 5 a is a graph of the fluorescence spectra of HITCI on SiO₂ C₁₈;

FIG. 5 b is a graph of the fluorescence spectra of HITCI on TiO₂;

FIG. 6 is a graph of the fluorescence spectra of Rhodamine 6G on silica;

FIG. 7 a is a graph of the relative fluorescence intensity at 720 nm ofTPP on TiO₂ as a function of initial fluorophore solution concentration;

FIG. 7 b is a graph of the fluorescence spectra of TPP on TiO₂: and

FIG. 8 is a schematic view of a field system suitable for use with thepresent invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Currently the most effective way to detect an IED is through visualobservation of the local environment and the population. This isaccomplished by looking for objects that appear new to the environmentor out of place, areas where the ground looks to have been recentlydisturbed and local people who appear nervous or disperse whenapproached. This visual observation method is a technique known aschange detection, that is, the identification of changes in theenvironment that either identify the TED directly, or identify tell-talecharacteristics associated with IED emplacement (e.g. digging, newobjects in the environment, new paths leading to an existing object,etc.). Change detection is a very powerful tool that can be used tocounter asymmetric threats, but to be maximally effective it must besensitive to subtle changes in a very noisy background while minimizingboth false positives and false negatives.

For IED identification in highly cluttered places, the most effectivechange detection technique is one that employs fading memory, i.e., awitness taggant whose signal fades (degrades) with time. A particularlysimple optical change detection scheme works by comparing cells withintwo images of a specific area, separated in time. The system may operateeither by detecting a disturbance in the deployed region or the presenceof a taggant in the non deployed region. Coincident detection of adisturbance and the presence of a taggant increases the probability thata disturbance has occurred. It is also possible to deploy taggantunderground. If for example a road surface is repaired and the taggantis deployed beneath the surface then the presence of taggant wouldindicate a disturbance. Casual inspection of the surface does not showthe presence of taggant. However wavelength and time resolvedfluorescence imaging makes the change readily apparent.

In a similar fashion one can look for changes between two scenes wherein a later scene something has been removed or altered. For example,with the taggant deployed on a road, the material fluoresces whenexcited with UV light. The material is dispersed uniformly across theroad and the fluorescence image and the temporally resolved image aretypical of a uniformly dispersed taggant. At a later time a hole is dugin the road which disturbs the fluorescent powder, and then the hole isrefilled, While it is difficult to see the change in the surface bycomparing images obtained before and after the disturbance is easilydetected. To “reset” the area, additional powder would be sprayed overthe road so that future “changes” can be detected. This can present aproblem if the taggant does not degrade with time (e.g. have a fadingmemory). As more and more taggant is dispersed, the ground becomescontaminated to a degree that the signal to noise ratio of the “changes”is so low that it becomes virtually impossible to detect. The bestsolution is to have a taggant that can be turned off at some presettime, for example just prior to re-spraying the area. Here each newapplication would be laid down on a pristine surface with no previoustagging contaminants. While a passive, binary (on/off) taggant is likelyimpossible to manufacture, it is possible to design the tagging compoundin such a way as to allow it to disappear over time, from hours toweeks.

Because the level of background “noise” exponentially increases in urbanor populated areas, the degradation time of the taggant should be varieddepending on the area of operation (AO). Since every AO has itscharacteristic IED tactics, a variable degradation time will allow thetaggant to be optimally tailored to maximize its effectiveness.

Additional advantageous taggant characteristics would be:

-   -   Extremely low manufacturing costs since the taggant will be        dispersed over large areas numerous times;    -   Easily deployed and provides uniform coverage using relatively        simple equipment (like paint sprayers or crop dusters);    -   Signal when viewed by appropriate sensing system must have a        good signal to noise ratio and good background contrast;    -   Taggant signal should be difficult to reverse engineer or should        provide a coded return (anti-spoof characteristic);    -   The taggant should be difficult to detect by the unaided eye.

I. Composition

The degradable taggant of the present invention is developed bycombining two low-cost, highly stable compounds: laser dyes and titaniumdioxide or silicon dioxide. Laser dyes are a desirable choice becausethey are readily available, come in a myriad of absorption/emissionwavelengths, have well defined fluorescence times, and can be chosen tohave a very low production cost. Titanium dioxide, or TiO₂ (commonlyfound in house paints, sun tan lotions, and tooth paste), is a whitepowder that is highly reflective, is a strong photocatalyst underultraviolet (UV) light, and because of its wide commercial use, isproduced in great quantities at very low cost. While both the laser dyesand TiO₂ are individually very stable, when TiO₂ is coated with a laserdye the compound becomes photoreactive, that its, it degrades over timewhen exposed to light through the process of photobleaching.

The laser dye coated TiO₂ taggant has a number of highly desirablecharacteristics for the proposed application:

-   -   Taggant is completely passive, both in generating a return        signal and in the method of degradation.    -   It will be nearly undetectable by the unaided eye making it        difficult to detect that an area has been treated.    -   Because it is a powder, it can be applied using conventional        spraying equipment. Additionally, it can be suspended in a        liquid such as water to minimize unwanted dispersion caused by        the wind.    -   Taggants can be created with a variety of absorption/emission        wavelengths. By mixing several taggant “colors” together a coded        signal can be created that makes spoofing virtually impossible        and provides other useful intelligence.    -   Fluorescence signal has two detection modes, absorption/emission        band separation and fluorescent lifetime. Sensing both enhances        SNR and adds an additional level of security.    -   An odorizer can be used to allow canine detection of the        taggant.    -   Laser dyes and sensing techniques can be selected to eliminate        sun blinding, allowing change detection sensing to occur both        day and night.

The taggants can be made of a number of different components,substrates, derivatizations of the substrate, dye combinations,antioxidants and other stability or spectral modifiers. Even with onlytwo dyes it is possible to modify all the other components to makeforgery of the particle difficult.

The substrate may be a semiconductor (titanium dioxide) or an insulator(silica or a polymer such as PVA or hydroxycellulose). The substrate maybe composed of gold nanoparticles (typically 300 nm in diameter). Bycontrolling the spacing between the dye and the nanoparticles thefluorescence intensity may be increased.

The substrate may be derivatized to control its ability to “solubilize”the dye on the surface and to give it the ability to stick to clothingor skin.

The dye combination and the concentration of dyes controls the emissionspectrum of the dyes and depending on the degradation of the taggant,changes with time. This change may be detected by monitoring therelative intensities of emission bands and ratioing the result or bymonitoring the fluorescence lifetimes of the dyes on the surface or by acombination of these methods. Ratiometric imaging and temporalmeasurements reduces the effect light scattering and increases thesecurity of the measurement and the selectivity of the measurement. Itis possible to fabricate a complex taggant using only two dyes but thecomplexity is increased dramatically by using three or more dyes.

Attachment of the dyes to a substrate particle may be throughphysisorption, electrostatic interactions or through covalentattachment. Electrostatic interactions and covalent attachment increasesthe difficulty of duplicating the taggant by an unauthorized individual.

Addition of antioxidants changes the stability of the dyes and alsomodifies the spectroscopy through a mechanism of photoelectron transfer.Incorporation of saturatable absorbers modifies the emission of thedyes. The saturatable absorber does not itself fluoresce but changes theemission spectrum of the taggant.

In one embodiment, a pressure sensitive taggant may be fabricated usinga crushable capsule. The capsule contains a solution of fluorophore anda quencher such as sodium iodide. The capsule does not fluoresce. Uponcrushing the capsule the solution leaks out, the solvent evaporates andthe fluorophore now fluoresces.

II. Method of Use

Once an area of interest has been tagged, activity in that area willresult in a change in the taggant's areal distribution. For example,digging will cause the taggant to be mixed with the excavated dirt. Whenthe dirt is replaced, the taggant's distribution in that area will besignificantly different both spatially and temporally. Here, changedetection, using ratiometric fluorescence spectroscopic techniques, canbe accomplished by either comparing scanned data from two differenttimes, or by performing a spatial correlation to look for adjacent areaswith significantly different intensities. This same change detectiontechnique can be used to detect objects added to an area of interestafter it has been tagged. Since the new object will not be covered bythe taggant, its return signal intensity will be zero. Again, timeseries comparisons or areal correlation techniques will quickly identifythe new object (even if an attempt is made to artificially tag theobject by pacing it in contact with objects in the area that are coveredwith taggant).

A very important characteristic of the taggant is that individualswalking around in a tagged area will not only alter the taggant'sdistribution (which will be detected), they will also pickup taggant ontheir shoes and clothing. Because the taggant fluoresces, and is coded,these individuals can be quickly identified and the specific AO wherethey performed activity located. Additionally, by adding an odorizer tothe taggant, canines can be used to track and locate individuals whowalked through a tagged area. Coded taggant found on the cloths ofindividuals that correlates to an AO where an IED was found or triggeredwould provide valuable physical evidence. The system of the presentinvention could be used by individuals on foot, operated from movingvehicles, and deployed on unmanned ground and air vehicles.

III. Properties

The degradable taggant of the present invention has the followingproperties: the taggant is a particulate which simplifies the depositionof the material. A combination of fluorophores is used which facilitatesratiometric measurement of the fluorescence and allows the particles tobe coded. By characterizing as few as five fluorophores it is possibleto combine three of these compounds in ten different ways, or two ofthese compounds in thirty different ways. Coding the taggant reduces theability to forge the particles and also allows for additionalintelligence data to be collected. The stability of the particles isdependent on the concentration of fluorophore applied to the particlesurface and to the presence of simple antioxidant molecules formulated)with the dyes. It is possible with this methodology to control thetimescale over which the taggant degrades.

The fluorophores chosen emit fluorescence in the red to near infra redregion of the electromagnetic spectrum. The eye is least sensitive tothis region of the spectrum, however, it is a region that is readilyimaged by commercial hyperspectral imaging devices.

Dye Selection

The dyes used in the present invention are selected from a groupconsisting of three commonly available red/NIR dyes and theirphotochemistry on titanium dioxide particles. These) aretetraphenylporphyrin (TPP), indodicarbocyanine (C5) andindotricarbocyanine (C7). The absorption and fluorescence spectra ofthese molecules are shown in FIG. 3. Other dyes which may be used may befound in Brackmann, U. (2000) “Lambdachrome® Laser Dyes”, Lambda PhysikAG•D-37079 Goettingen•Germany (which is incorporated by reference hereinin its entirety).

Emission and excitation spectra for a total of fourteen dyes have beenobtained. These spectra have been acquired in solution, adsorbed ontoTiO₂ particles and on Silica particles. Some spectra have also beenacquired on the derivatized particles.

The structures and the emission spectra of these dyes are shown below:

CAS Number: 41830-80-2 Molecular Weight: 361.74 g/mol Molar AbsorptionCoefficient: 6.74 × 10⁴ L mol⁻¹ cm⁻¹ @ 601 nm

Emission in ethanol at 625 nm, no fluorescence observed from TiO₂

CAS Number: 19764-96-6 Molecular Weight: 536.50 g/mol Molar AbsorptionCoefficient: 21.5 × 10⁴ L mol⁻¹ cm⁻¹ @ 741 nm

Emission spectra for HITCI on C₁₈ derivatized silica and on TiO₂(λ_(exc) 665 nm) are shown in FIGS. 6 a and b. HITCI on bare SiO₂ isblue shifted by about 20 nm compared to the C₁₈ derivatized substrate.

CAS: 63561-42-2 Molecular Weight: 538.95 g/mol Molar AbsorptionCoefficient: 9.25 × 10⁴ L mol⁻¹ cm⁻¹ @ 643 nm

Emission in ethanol at 670 nm, no fluorescence observed from TiO₂

CAS: 87004-02-2 Molecular Weight: 378.85 g/mol Molar AbsorptionCoefficient: 3.80 × 10⁴ L mol⁻¹ cm⁻¹ @ 480 nm

Emission in ethanol at 660 nm, no fluorescence observed from TiO₂

CAS: 76433-29-9 Molecular Weight: 434.94 g/mol Molar AbsorptionCoefficient: 6.15 × 10⁴ L mol⁻¹ cm⁻¹ @ 570 nm

Emission in ethanol at 730 nm, no fluorescence observed from TiO₂

CAS: 76433-25-5 Molecular Weight: 527.96 g/mol Molar ExtinctionCoefficient: 5.05 × 10⁴ L mol⁻¹ cm⁻¹ @ 585 nm

Emission in ethanol at 790 nm, no fluorescence observed from TiO₂

CAS: 120528-73-6 Molecular Weight: 513.96 g/mol Molar AbsorptionCoefficient: 5.05 × 10⁴ L mol⁻¹ cm⁻¹ @ 585 nm

Emission in ethanol at 790 nm, no fluorescence observed from TiO₂

CAS: 53340-16-2 Molecular Weight: 417.85 g/mol Molar AbsorptionCoefficient: 7.75 × 10^(∝1) L mol⁻¹ cm⁻¹ @ 633 nm

Emission in ethanol at 670 nm, no fluorescence observed from TiO₂

CAS: 23178-67-8 Molecular Weight: 609.17 g/mol Molar AbsorptionCoefficient: 23.1 × 10⁴ L mol⁻¹ cm⁻¹ @ 780 nm

Emission in ethanol at 820 nm, also fluoresces on TiO₂

CAS: 62669-60-7 Molecular Weight: 431.87 g/mol Molar AbsorptionCoefficient: 9.20 × 10⁴ L mol⁻¹ cm⁻¹ @ 627 nm

Emission in ethanol at 645 nm, no fluorescence observed from TiO₂

CAS: 24796-94-9 Molecular Weight: 423.90 g/mol Molar AbsorptionCoefficient: 13.0 × 10⁴ L mol⁻¹ cm⁻¹ @ 646 nm

Emission in ethanol at 660 nm, no fluorescence observed from TiO₂

CAS: 989-38-8 Molecular Weight: 606.71 g/mol Molar AbsorptionCoefficient: 10.5 × 10⁴ L mol⁻¹ cm⁻¹ @ 533 nm

Emission in ethanol at 550 nm, weak fluorescence observed from TiO₂Emission on silica and C18 silica acquired at λ_(exc) 480 nm is shown inFIG. 7.

CAS: 60311-02-6 Molecular Weight: 606.71 g/mol Molar AbsorptionCoefficient: 10.6 × 10⁴ L mol⁻¹ cm⁻¹ @ 578 nm

Emission in ethanol at 590 nm, no fluorescence observed from TiO₂

CAS: 917-23-7 Molecular Weight: 614.74 g/mol Molar AbsorptionCoefficient: 18.9 × 10³ L mol⁻¹ cm⁻¹ @ 415 nm

The fluorescence from derivatized TiO2 can be as much as four timeshigher than the underivatized substrate. Fluorescence intensitymeasurements are not absolute; they require reference to a standardmaterial. (See FIGS. 7 a and 7 b).

Absorption of Dyes onto Substrate

Several methods have been used to adsorb dyes onto the surface ofsubstrates.

Titanium dioxide (TiO₂ 98% rutile, 0.5 gram, 5 μm) was mixed with 4 mLof deionized water. To this mixture was added 1 mL of ethanolic dyesolution and shaken vigorously for 3 hours. Mixture was vacuum filteredand vacuum dried for 4 hours at 110° C. and 30 in Hg. The dry powder wasstored in an amber vial in a dark cabinet until characterized. Dyeconcentrations used were 10-100 μM. TPP was added to the powder slurryin ethyl acetate.

Silicon dioxide (SiO₂ 0.5 gram, 40 μm) was mixed with 4 mL of ethanol.To this mixture 1 mL of ethanolic dye solution was added and shakenvigorously for 3 hours. Mixture was vacuum filtered and vacuum dried for4 hours at 65° C. and 30 in Hg. The dry powder was stored in an ambervial in a dark cabinet until characterized. Dye concentrations used were10-100 μM. TPP was added to the powder slurry in ethyl acetate.

Modification of Substrate

Titanium dioxide particles were derivatized with an eighteen carbonalkane to render them hydrophobic. Alkanes of varying numbers of carbonscan also be used to modify titanium) dioxide of silicon dioxideparticles. TiO₂ particles (5 μM) were plasma cleaned in oxygen plasmafor three minutes using a Plasma Prep II plasma cleaner. The particleswere immediately placed in a vacuum desiccator with a watch glass with˜0.5 mL octadecyltrichlorosilane. The desiccator was sealed andevacuated using a house vacuum overnight. The resulting particles werecharacterized using ATR-FTIR and stored in a sealed vial until usage.

C₁₈ derivatized SiO₂ is commercially available as a packing material forreverse phase chromatography. Both TiO₂ and SiO₂ were treated with dyesas previously described. The derivatized particles are extremelyhydrophobic and repel water which can be seen in the photograph of FIG.2.

FIG. 2 shows SiO₂—C₁₈ with Rhodamine 6G in the presence of waterdroplets. The powder forms a coating on the droplets, but will notdisperse in the water. It is expected that this property will facilitatebinding of the particles to skin and clothing and will be difficult towash off.

Advanced Taggant

The physisorbed taggant on silica already developed and described aboveis suitable for a number of applications and can be used to test theimaging system. An advanced taggant uses specific attachment of dyematerial to the substrate. This material will consist of multiple layersof fluorophores and antioxidants. The spacing between these layers iscontrolled using linker molecules. Linker molecules comprise metalbinding ligands connected by a short alyl chain, for example4,4′-(ethane-1,2-diylbis(oxy))dipyridine-2,6-dicarboxylic acid. Thissystem can be implemented either on silica or on gold nanoparticles.

The advanced taggant will have enhanced fluorescence, greater stabilityand features that make counterfeiting more difficult. The use of selfassembled multilayers simplifies the design of complex multidimensionalsystems. Fabricating the taggant on gold nanoparticles has the potentialto increase the stability of the taggant and the quantum yield offluorescence. The use of gold nanoparticles does not significantlyincrease the cost of the taggant system. We have calculated that oneounce of 300 nm gold nanoparticles, deployed at a density of 1 particleper square millimeter, would yield a strip 2 meters wide and more than50,000 kilometers long. In short, the gold is not a significant costcomponent of the taggant.

The advanced taggant uses controlled spacing between fluorophores tomodulate energy and electron transfer. The technology is described inU.S. Pat. No. 6,893,716 “Non-covalent assembly of multilayer thin filmsupramolecular structures” by William Grant McGimpsey and John C.Macdonald, the entire contents of which are hereby incorporated byreference herein. The fluorophores are similar to those that are usedfor the formation of the physisorbed taggant but are derivatized with achemistry that will bind metal ions such as chalidamic acid as describedin U.S. Pat. No. 6,893,716 and controls electron and energy transferbetween components. As a result the same fluorophores may generate awide range of spectra with different stability. FIG. 3 shows a schematicof a three layer assembly. The fluorophores are represented by thecolored circles. The M represents the non covalent linkage betweenmolecules. The chemistry may be assembled on gold, silica, glass orpolymer substrates.

Acquisition of Spectra

Fluorescence spectra have been acquired using a modified cuvette shownin FIG. 4. The sample (typically 0.5 gram) is contained in an inner tubewithin a conventional cuvette. Fluorescence is collected at an angle of90° to the excitation to minimize scattered light entering the detector.

Particle Labeling

One method to label TiO₂ particles is to use hydrophobic andelectrostatic interactions. Dyes are dissolved in an organic solvent andmixed with TiO₂ particles. The particles are stirred in the solution for12 hours and then filtered, washed and dried. The fluorescence of theparticles is measured in a powder cell. Both excitation and emissionspectra are collected to determine the optimum excitation wavelength.The stability of the dye/particle combination is determined by measuringthe excitation and emission spectra as a function of irradiation time.The irradiation source is a 250 W Xenon arc lamp filtered to removewavelengths below 300 nm. The sample holder is jacketed so that thetemperature of the sample may be controlled from 4-100° C., Adsorptionof dyes onto TiO₂ particles has been widely used to constructphotochemically active systems for solar energy conversion. In thisapplication, the same techniques are being used to produce dye particlesbut with the aim of minimizing or controlling the interaction of lightwith the system. Physisorption of the dye onto the particle surfaceprovides a rapid, economical route to creating relatively complexsystems. The technique of fabrication of chemically derivatized surfacesmay also be used. This technology allows for more complex combinationsof dyes and other materials to be precisely controlled. The physicaladsorption of dyes onto the surface is simple and because the sample isa solid, diffusion kinetics need not be considered in designing stablefluorescent entities.

Fluorescence Measurement

The fluorescence properties of the particles are monitored using bothsteady state and time resolved instrumentation. The steady statecharacterization uses a Perkin Elmer LS-50B Spectrofluorometer.Following the development of promising dye/particle systems, timeresolved fluorescence spectra is obtained using a Photon TechnologyInternational TM3/2005 equipped with a dye pumped nitrogen laser. Theinstrument is capable of determining lifetimes as short as 100 ps. Thisis an important tool for determining combinations and concentrations ofdyes and lifetime modifiers as well as TiO₂ crystal type, i.e. rutileand anatase.

Photostability

The average power of the sun that reaches the earth in certain climaticconditions is approximately 300 W/m². Of this energy about 50% is in thevisible region of the electromagnetic spectrum. Therefore, the incidentpower of solar radiation is about 15 mW/cm². Laser dyes are designed tobe stable at powers significantly greater than this. For example, testshave been done on the stability of a number of laser dyes at peak laserintensities of 20-100 kW/cm². The measured quantum yields of degradationare around 1×10⁻⁶ dye molecules bleached per photon absorbed. In otherwords, it would take a million absorbed photons to destroy a dyemolecule.

Using these figures it is possible to estimate to within an order ofmagnitude the stability of the dyes. The taggant is considered to bemade up of a single molecular film of dye. Such films made up of a dyewith a molar absorption coefficient of 10⁵ M⁻¹ cm⁻¹ has an absorption ofabout 0.01. This means that the film will absorb about 2% of the lightincident on it. If the wavelength of the average visible photon is 500nm, then the number of photons incident at 15 mW/cm² will be 3.78×10¹⁶.Only 2% of these will be absorbed by the film so there will be 7.56×10¹⁴photons absorbed. If the quantum yield of degradation is 10⁻⁶ then itwill take 10⁵ seconds or about 28 hours for 10% of the dye to degrade.

There are a number of factors which will affect this estimate. Thequantum yield of degradation is given for solution phase chemistry;solid films are likely to be significantly more stable. If the averagedaytime temperature is about 120° F. (48° C.), dyes are likely to beless stable at such elevated temperatures.

Photostability studies can be carried out by irradiating samples withbroadband light to simulate solar radiation at a range of powerscentered around 15 mW/cm² and at room temperature (20° C.) and atelevated temperatures. The ideal taggant will degrade at a constant rateirrespective of the light that falls on it and the temperature to whichit is subjected. The taggant will be most sensitive to the amount oflight it is irradiated with and less so to the temperature. This isbecause in solid phase chemistry there is little or no diffusion whichis the factor that makes solution phase chemistry most sensitive totemperature. By carefully controlling the spacing of the dyes on theparticle surface one can determine the processes that lead to dyedegradation and dye stability.

Control of Stability

Following absorption of a photon a molecule is promoted to an excitedstate. The processes that occur following excitation determine thestability of the dye. Fluorescence or emission of heat (non radiativedecay) return the dye to the ground state and leaves the dye unaltered.For a dye that is in the solid state, the processes that can lead todestruction of the dye are reaction of the excited state with anotherexcited state or reaction with a ground state molecule. It is alsopossible to control the stability of a dye by colocalizing the dye withantioxidants or redox active materials. Finally, there are two commonlyavailable crystalline forms of TiO₂. These are rutile and anatase TiO₂.The rutile form is relatively photochemically inert and leads to a morestable dye/particle complex. There are a number of strategies to controlthe length of time that a dye on a TiO₂ particle remains fluorescent.

A range of dye/particle mixtures are available that have a predictabledegradation rate. Because the temperature and intensity of the sunlightvary considerably between the winter and summer months, the taggant isproduced in two variants suitable for each season. Since the primarydegradation mechanisms involve photochemical and thermal mechanisms itis also possible to utilize these taggants in the “wrong” season as longlife and short life taggants respectively.

The approaches to control the lifetime of the taggant are summarized inTable 1:

TABLE 1 Increase Stability Decrease Stability Rutile TiO₂ Anatase TiO₂Low dye concentration High dye concentration Different dyes on differentparticles Co-localize dyes Co-localize antioxidants Co-localize redoxactive agents

Radiometric Monitoring

Monitoring of fluorescence intensity alone is problematic sincefluorescence is not an absolute technique and cannot be quantitatedunless compared to a standard fluorescing material. There are two widelyaccepted approaches to this problem. The first is to utilize timeresolved spectroscopy and measure the excited state lifetime. The secondis to use a combination of dyes and ratio the relative intensities ofthe two dyes.

Monitoring the excited lifetime increases the complexity of themonitoring system but gives extremely reliable data. Commerciallyavailable hyperspectral imaging systems may be used to image thefluorescent taggant and ascertain its authenticity by comparing therelative intensity of different regions of the spectrum throughratiometric means.

Toxicity

A critical factor in spraying taggants is that there is no environmentalimpact of the material. The choice of designing unique, fluorescent,coded systems with TiO₂ is deliberate. TiO₂ is widely used today in theenvironment at high concentrations in the presence of a wide range ofdyes. TiO₂ is used in cosmetics, paints, as a filler in drugformulations and as a whitening agent for paper. The dyes used arepresent at low concentration and many of them are actually related tofood dyes as well as other coloring applications. The application of thematerial is at low concentration, thereby limiting the environmentalimpact.

Transfer of Taggant to Individuals

The ability to chemically modify the surface of the particles allows oneto make the surface hydrophobic or hydrophilic, electrostaticallypositive or negative. These properties may be exploited to provideintelligence data of routes to and from the suspected IED threat regionby designing the dye/particle to adhere to an individual who tamperswith the taggant. Alternatively, where individuals are involved in anambush scenario, their vehicles may be deliberately tagged to preventthem from disappearing into civilian crowds.

IV. Method of Making

Three dyes are formulated with TiO₂ particles in the anatase form andthe rutile form. This is achieved by dissolving the dyes in ethanolsolution and stirring them with the TiO₂ powder for twelve hours. Thesamples are then filtered, washed and dried. The excitation and emissionspectra of these six samples are measured and then the samplesirradiated using a xenon arc lamp at an average power of 15 mW/cm². Thedecomposition of the dyes are then determined from the change in theexcitation and emission spectra of the samples. In this way, thesimplest formulations of dye and TiO₂ are assessed for stability. Thedye-rutile samples are significantly more stable than the dye-anatasesamples. The temperature dependence is also determined at an elevatedtemperature of 50° C.

The ratiometric monitoring of the fluorescence excitation and emissionspectra of different dye-TiO₂ particle mixtures is demonstrated. Withindividual dyes located on different particles there is no interactionsof different dyes. Different dyes decay at different rates on the TiO₂particles and therefore monitoring of the relative intensities of thetwo dyes allows one to measure the length of time a powder mixture hasbeen deployed.

First, two dyes are co-localized on the same particle (coding) followedby the co-localization of antioxidants and redox active agents to finelycontrol the lifetime of the taggant.

The co-localization of dyes onto the same particle effectively creates anew fluorescent system that can only be duplicated by following thepreparation methods exactly. This is because dye-dye interactions causeenergy and electron transfer pathways to be created in these systems.The result is that the system has a fluorescence excitation and emissionspectrum which is dependent on how the dyes are localized on the surfaceof the particles. These interactions are dependent on the concentrationsof the dyes when applied to the TiO₂ powder and geometry of the particlesurface. Unless the source of the TiO₂ can be duplicated it is verydifficult to mimic or forge the photophysical behavior of the dyeparticle system.

Precise control of the degradation time of the fluorescent taggant iscontrolled by incorporation of antioxidant and redox active materials.The primary mechanism by which dyes photobleach is electron transfer.Co-localization of antioxidants such as butylated hydroxytoluene (BHT)with the dye TiO₂ can quench electron transfer reactions therebyimproving the lifetime of the dye. BHT is a useful antioxidant for thispurpose since it does not decompose on exposure to air. It is commonlyapplied to the packaging of foodstuffs to prevent oxidation. It has beenproposed that BHT is effective in this role since it only reacts withstrong oxidants and is therefore ideal in a photoreactive system.

Design of the Imaging System

Detection of fluorescence light will be achieved using a large optic(100 mm aperture). It is estimated that for a single fully loaded 40 μmtaggant particle, at a distance of 3 meters, with photons emitted fromevery particle the lens will collect 80,000 photons. This is readilydetectable using phase sensitive detection which allows for timeresolved data to be collected.

The detector will be a photomultiplier tube coupled to a high throughputspectrograph (in house). The output of the PMT is connected to an RFlock-in amplifier and thence to a computer. Analysis of the data will becarried out for phase shift and depth of modulation which are bothrelated to the lifetime of the fluorophore.

Concept of Employment

The schematic of FIG. 8 summarizes how the system might be used in thefield. The system may operate either by detecting a disturbance in thedeployed region or the presence of a taggant in the non deployed region.Coincident detection of a disturbance and the presence of a taggantincreases the probability that a disturbance has occurred. It is alsopossible to deploy taggant underground. If for example a road surface isrepaired and the taggant is deployed beneath the surface then thepresence of taggant would indicate a disturbance.

The material will be deployed in a liquid to enable targeted, precisedistribution and to prevent loss of material due to wind. The coverageis extremely low with a typical density of 1 particle per squarecentimeter. A low coverage density decreases the probability that thematerial could be swept up in quantities large enough to be analyzed byan enemy. The detector consists of an excitation laser and a detector.The laser is scanned across the ground up to 300 meters in front of theconvoy. Both back scattered UV light and fluorescence is detected usingavalanche photodiodes. The excitation source is modulated which allowstemporal data to be detected. The UV scattered light is monitored sothat changes in the temporal profile of the excitation source and theemission may be compared. This information is used to calculate theapparent fluorescence lifetime of the irradiated area.

1. A method of identifying changes comprising: dispersing a taggant;detecting a taggant signal; recording a first signal; detecting a secondsignal; recording said second signal; comparing said first signal tosaid second signal; identifying differences between said first signaland said second signal.
 2. The method of claim 1, wherein said taggantsignal fades or degrades over time.
 3. The method of claim 1, whereinsaid taggant is a composition comprising one or more fluorophores and asubstrate.
 4. The method of claim 1, wherein said taggant is notdetectable by the unaided eye.
 5. The method of claim 1, wherein saidtaggant further comprises an odorant.
 6. The method of claim 1, whereinsaid taggant comprises one or more fluorophores and a substrate.
 7. Themethod of claim 6, wherein said one or more fluorophores is a laser dye.8. The method of claim 7, wherein said one or more dye is selected fromthe group consisting of tetraphenylporphyrin, indodicarbocyanine,indotricarbocyanine, HITCI, HITCP, and Rhodamine 6G.
 9. The method ofclaim 6, wherein said substrate is metal nanoparticles, a semiconductoror an insulator.
 10. The method of claim 6, wherein said substratefurther comprises an alkane.
 11. The method of claim 9, wherein saidsemiconductor is titanium dioxide.
 12. The method of claim 9, whereinsaid insulator is selected from the group consisting of gold, silica,PVA and hydroxycellulose.
 13. The method of claim 1, wherein saidtaggant further comprises an antioxidant or a redox active material. 14.The method of claim 13, wherein said antioxidant is butylatedhydroxytoluene.
 15. The method of claim 1, wherein said dispersing is byspreading, spraying, or dusting.
 16. A composition comprising one ormore fluorophores and a substrate.
 17. The composition of claim 16,wherein said one or more fluorophores is a laser dye.
 18. Thecomposition of claim 17, wherein said one or more dye is selected fromthe group consisting of tetraphenylporphyrin, indodicarbocyanine,indodicarbocyanine, HITCI, HITCP, and Rhodamine 6G.
 19. The compositionof claim 16, wherein said substrate is a metal, semiconductor or aninsulator.
 20. The composition of claim 16, wherein said substratefurther comprises an alkane.
 21. The composition of claim 19, whereinsaid semiconductor is titanium dioxide.
 22. The composition of claim 19,wherein said insulator is selected from the group consisting of silica,PVA and hydroxycellulose.
 23. The composition of claim 16, furthercomprising an antioxidant or a redox active material.
 24. Thecomposition of claim 23, wherein said antioxidant is butylatedhydroxytoluene.